This invention relates generally to woven fabrics, and relates more specifically to woven fabrics for papermakers.
In the conventional fourdrinier papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper xe2x80x9cstockxe2x80x9d) is fed onto the top of the upper run of an endless belt of woven wire and/or synthetic material that travels between two or more rollers. The belt, often referred to as a xe2x80x9cforming fabricxe2x80x9d, provides a papermaking surface on the upper surface of its upper run which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium, thereby forming a wet paper web. The aqueous medium drains through mesh openings of the forming fabric, known as drainage holes, by gravity alone or with assistance from one or more suction boxes located on the lower surface (i.e., the xe2x80x9cmachine sidexe2x80x9d) of the upper run of the fabric.
After leaving the forming section, the paper web is transferred to a press section of the paper machine, in which it is passed through the nips of one or more pairs of pressure rollers covered with another fabric, typically referred to as a xe2x80x9cpress felt.xe2x80x9d Pressure from the rollers removes additional moisture from the web; the moisture removal is often enhanced by the presence of a xe2x80x9cbattxe2x80x9d layer on the press felt. The paper is then conveyed to a drier section for further moisture removal. After drying, the paper is ready for secondary processing and packaging.
Typically, papermaker""s fabrics are manufactured as endless belts by one of two basic weaving techniques. In the first of these techniques, fabrics are flat woven by a flat weaving process, with their ends being joined to form an endless belt by any one of a number of well-known joining methods, such as dismantling and reweaving the ends together (commonly known as splicing), or sewing a pin-seamable flap on each end or a special foldback, then reweaving these into pin-seamable loops. In a flat woven papermaker""s fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross machine direction. In the second technique, fabrics are woven directly in the form of a continuous belt with an endless weaving process. In the endless weaving process, the warp yarns extend in the cross machine direction and the filling yarns extend in the machine direction. As used herein, the terms xe2x80x9cmachine directionxe2x80x9d (MD) and xe2x80x9ccross machine directionxe2x80x9d (CMD) refer, respectively, to a direction aligned with the direction of travel of the papermaker""s fabric on the papermaking machine, and a direction parallel to the fabric surface and traverse to the direction of travel. Both weaving methods described hereinabove are well known in the art, and the term xe2x80x9cendless beltxe2x80x9d as used herein refers to belts made by either method.
Effective sheet and fiber support and an absence of wire marking are typically important considerations in papermaking, especially for the forming section of the papermaking machine, where the wet web is initially formed. Wire marking is particularly problematic in the formation of fine paper grades, as it can affect a host of paper properties, such as sheet mark, porosity, xe2x80x9csee throughxe2x80x9d and pin holing. Wire marking is typically the result of individual cellulosic fibers being oriented within the paper web such that their ends reside within gaps between the individual threads or yarns of the forming fabric. This problem is generally addressed by providing a permeable fabric structure with a coplanar surface that allows paper fibers to bridge adjacent yarns of the fabric rather than penetrate the gaps between yarns. As used herein, xe2x80x9ccoplanarxe2x80x9d means that the upper extremities of the yarns defining the paper-forming surface are at substantially the same elevation, such that at that level there is presented a substantially xe2x80x9cplanarxe2x80x9d surface. Accordingly, fine paper grades intended for use in quality printing, carbonizing, cigarettes, electrical condensers, and like grades of fine paper have typically heretofore been formed on very finely woven or fine wire mesh forming fabrics.
Typically, such finely woven fabrics include at least some relatively small diameter machine direction or cross machine direction yarns. Regrettably, however, such yarns tend to be delicate, leading to a short surface life for the fabric. Moreover, the use of smaller yarns can also adversely effect the mechanical stability of the fabric (especially in terms of skew resistance, narrowing propensity and stiffness), which may negatively impact both the service life and the performance of the fabric.
To combat these problems associated with fine weaves, multi-layer forming fabrics have been developed with fine-mesh yarns on the paper forming surface to facilitate paper formation and coarser-mesh yarns on the machine contact side to provide strength and durability. For example, fabrics have been constructed which employ one set of machine direction yarns which interweave with two sets of cross machine direction yarns to form a fabric having a fine paper forming surface and a more durable machine side surface. These fabrics form part of a class of fabrics which are generally referred to as xe2x80x9cdouble layerxe2x80x9d fabrics. Similarly, fabrics have been constructed which include two sets of machine direction yarns and two sets of cross machine direction yarns that form a fine mesh paper side fabric layer and a separate, coarser machine side fabric layer. In these fabrics, which are part of a class of fabrics generally referred to as xe2x80x9ctriple layerxe2x80x9d fabrics, the two fabric layers are typically bound together by separate stitching yarns. As double and triple layer fabrics include additional sets of yarn as compared to single layer fabrics, these fabrics typically have a higher xe2x80x9ccaliperxe2x80x9d (i.e., they are thicker than) comparable single layer fabrics. An illustrative double layer fabric is shown in U.S. Pat. No. 4,423,755 to Thompson, and illustrative triple layer fabrics are shown in U.S. Pat. No. 4,501,303 to Osterberg, U.S. Pat. No. 5,152,326 to Vohringer, and U.S. Pat. No. 5,437,315 to Ward.
One particularly desirable type of triple layer fabric is illustrated in U.S. Pat. No. 5,967,195 to Ward. The fabrics described therein include pairs of stitching yarns between adjacent top CMD yarns that alternately interweave with the top and bottom MD yarns of the fabric. They do so in such a manner that they xe2x80x9ccomplete the weavexe2x80x9d of the weave pattern of the top MD and top CMD yarns. Such a papermaking surface can provide good fiber support, drainage and interlaminar wear resistance. Alternative fabrics of this type are illustrated in U.S. Pat. No. 5,826,627 to Seabrook et al. However, these fabrics can have relatively high caliper, which can have a negative impact on water carry and fiber carry, increasing both of these properties.
The foregoing demonstrates that it would be desirable for a papermaker""s forming fabric to have a balance of properties important to papermaking, including relatively low caliper, low void volume for drainage purposes, and good fiber support. It would be particularly desirable for such a forming fabric to have a triple layer structure.
The present invention, which is directed to a triple layer papermaker""s fabric, can provide these desirable characteristics. The triple layer forming fabric includes: a set of top machine direction yarns; a set of top cross machine direction yarns interwoven with the top machine direction yarns to form a top fabric layer; a set of bottom machine direction yarns; a set of bottom cross machine direction yarns interwoven with the bottom machine direction yarns to form a bottom fabric layer; and a plurality of stitching yarns interwoven with the top and bottom fabric layers. A pair of first and second stitching yarns is positioned between adjacent pairs of top cross machine direction yarns; the first and second stitching yarns of each pair are interwoven with the top and bottom machine direction yarns such that, as a fiber support portion of the first stitching yarn is interweaving with the top machine direction yarns, a binding portion of the second stitching yarn is positioned below the top machine direction yarns, and such that as a fiber support portion of the second stitching yarn is interweaving with the top machine direction yarns, a binding portion of the first stitching yarn is positioned below the top machine direction yarns. The first and second stitching yarns cross each other as they pass below a transitional top machine direction yarn, and each of the binding portions of the first and second stitching yarns passes below at least one of the bottom machine direction yarns. The top machine direction yarns, top cross machine direction yarns, and fiber support portions of the stitching yarns interweave to form a plain weave surface. The top machine direction yarns have a first diameter, the bottom machine direction yarns have a second diameter, and the top cross machine direction yarns have a third diameter, and a ratio of the first diameter and the second diameter is between about 0.75 and 0.95, and a ratio between the first diameter and the third diameter is between about 0.8 and 1.1. In this configuration, the yarns of the fabric can interweave, and the top and bottom layers of the fabric can intermesh and nest, such that the caliper and the void volume of the triple layer fabric are relatively low, yet the fiber support provided to paper stock is relatively high. As a result, the fabric can provide a desirable combination of properties in a triple layer design.
In certain preferred embodiments, a stitching yarn pair is positioned between each adjacent pair of top cross machine direction yarns. Also, in some embodiments the number of top and bottom cross machine direction yarns are the same, and in other embodiments the number of (a) top cross machine direction yarns and stitching yarn pairs and (b) bottom cross machine direction yarns are the same.
It is also preferred that the diameter of the top machine direction yarns is between about 0.12 and 0.14 mm, the diameter of the bottom machine direction yarns is between about 0.15 and 0.18 mm, and the diameter of the top cross machine direction yarns is between about 0.11 and 0.13 mm.
Objects of the present invention will be appreciated by those of ordinary skill in the art from a reading of the Figures and the detailed description of the preferred embodiments which follow, such description being merely illustrative of the present invention.